In case an obstacle with high reflection intensity (a truck, a guardrail, a shutter, a refrigerator, or the like) is present in the vicinity of a target entity such as communication equipment, electromagnetic interference occurs due to multipath propagation. This degrades communication quality. A phased-array antenna is known as a technology for transmitting or receiving electromagnetic waves selectively in a specific direction by sweeping a narrow-angle beam. The phased-array antenna including plural antenna elements can sweep a beam by actively changing phase planes of electromagnetic waves coming from the respective antenna elements. As a method for achieving the active change, a variable phase shifter is provided for the respective antenna elements, and independently controlled in order to obtain a desired beam angle. As a method of realizing a phased-array antenna that does not use the variable phase shifters, the antenna elements are connected via a Rotman lens capable of performing division and synthesis on an electromagnetic wave.
A conventional phased-array antenna using a Rotman lens can, as described in Japanese Patent Application Laid-Open Publication No. 2003-152422, avoid multipath propagation, which is caused by an obstacle, by producing a narrow-angle antenna beam which is formed by numerous antenna elements, and transmitting or receiving an electromagnetic wave to or from communication equipment present in a desired direction.
Japanese Patent Application Laid-Open Publication No. 2010-074781 describes that a Rotman lens having two input ports is used to construct a phased-array antenna. When power is fed to the two input ports, respective variable amplifiers are used to control a power ratio. Thus, a beam can be controlled at infinite stepped angles within an intermediate range between beams inputted through the input ports.
As described in Japanese Patent Application Laid-Open Publication No. 2005-354388, an antenna having a null point is known as a means for narrowing a beam (realizing a high resolution) without an increase in the number of antenna elements constituting a phased-array antenna. In case a monopulse radar is formed using the antenna that has the null point, a delta (Δ)-pattern beam and a sigma (Σ)-pattern beam are needed. As for a monopulse method, when two antennas are disposed sideways, although a signal propagating along a bisector of the two antennas is received at the same phase because distances to the antennas are identical to each other, a signal propagating in any other area undergoes a phase difference because of distance difference. Therefore, sum and difference signals of powers received by the two antennas are produced, and an incoming angle is estimated based on an amplitude ratio between the signals. In particular, the difference signal exhibits an abrupt amplitude change at a point near the null point. Therefore, when a phased-array antenna beam is swept, if an extreme value of the ratio of the sum and difference signals is measured concurrently, a radar offering a high angular resolution can be realized.